1. Field of the Invention
This invention relates to a method and apparatus for dewatering sludges and slurries, such as sewage sludges and paper mill slurries.
2. Prior Art
It is known to dewater sludges by placing the sludge on perforate plates or screens which may be inclined and which are supported above drainage plates, which may also be inclined. Examples of such dewatering apparatus appear in U.S. Pat. Nos. 1,537,818 and 596,540. It has also been observed that sludge can be dewatered and sun-dried by merely spreading the sludge upon a sloped slab such as a concrete slab. Thus, it has been known to separate two such slabs, which are sloped oppositely to form a trough, by a sand filter to which the water drained from the sludge is permitted to flow. Other and different types of techniques for dewatering sludge and the like appear in the following U.S. Pat. Nos.: 1,673,728, 2,259,688, 2,928,548, 1,940,952, 2,491,912, 3,662,896.
British Pat. No. 21,355 illustrates a dewatering technique applied to the withdrawal of wort from a mash tun.
U.S. Pat. No. 3,216,569 illustrates a sludge dewatering apparatus which has received attention recently in a number of U.S. sewage plants. In the technique described in this last named patent, sewage sludge is deposited onto a perforate screen or plate which is immersed into water, characterized as support water, such water supported in a chamber located below the perforate plate. After the sludge has been accumulated above the perforate plate to a desired depth, the support water located in the chamber underlying the perforate plate is slowly removed at a controlled rate. The slow water removal avoids a discharge of the sludge being dewatered through the perforations in the perforate plate. Consequently, a large fraction of the water normally present in the sludge is withdrawn from the sludge through the perforate plate with a minimum of escape of sludge. The sludge lying immediately upon the perforate member thus functions as a filter, which minimizes a downflow of sludge through the perforate member.